Sodium vapor arc discharge lamps for use in street lighting, for example, are well known. Because the sodium ionizes at relatively high temperature, the lamps usually include a gas such as neon or xenon which ionizes readily and forms an inital arc. This arc is used to initially ionize mercury vapor and the arc from the mercury vapor in turn ionizes the sodium. The start-up process for a cold lamp is typically in the order of five minutes.
The high voltage potential necessary to initiate an arc in the low pressure gases of the sodium lamp is derived from a ballast transformer connected to a constant current power source. In a typical installation, the ballast transformers for a number of lamps are connected in series across a moving coil type constant current power transformer. Because the transformer secondary of the ballast transformers looks at a very high impedance load until the arc is struck in the lamp, the voltage across the primary and secondary of the ballast transformer is initially quite high. With the rise in voltage with constant current, the power input to each lamp is therefore at a peak during initial start. The constant current transformer must therefore have a volt-ampere rating that is approximately four times the rating required to operate the lamps after they reach steady state operating condition. Since it is desirable to operate as many lamps as possible off of one power transformer, the relatively large start-up power requirement becomes a limiting factor. If the starting of the lamps could be staggered so that only a portion of the lamps were in the start-up mode at one time, the rating of the constant current transformer could be reduced or more lamps could be used on the same capacity circuit.
One solution which has been heretofore proposed is to provide a timed switch in association with a lamp which initially short circuits the primary of the ballast transformer and provides a shunt current path for the constant current from the power transformer. By opening the switches at delayed time intervals, groups of lamps could be activated at staggered time intervals. Several problems have limited the usefulness of such an approach. While mechanical contacts are used for the switches, because each set of contacts must handle the full current capacity of the system, contact life is quite limited and maintenance is difficult and expensive. Moreover, with shorting of the primary no voltage potential exists at the ballast unit to power a timing circuit. Mechanical or battery powered timers have been used but also present a maintenance problem.